Study on Chemical Reaction Mechanisms for Multi-component Diesel Surrogate Fuels and Oxygenated Biofuels
Author:Xu Zheng Xin
Supervisor:liu jing ping
Increasingly stringent fuel consumption and emissions regulations put forward higher demands on the performance of internal combustion engines.As renewable alternative fuels,biofuels can not only ease the energy shortage situation,but also significantly reduce soot emissions of diesel engine.Therefore,it is necessary to study the combustion process and soot formation mechanism of biofuel in diesel engine.The soot formation process is very complicated,involving both gas-phase chemical reaction kinetics and particle dynamics.It is a challenging task to simulate the soot formation process of diesel engine.The simulation method of CFD coupled chemical reaction kinetics is an important approach to study and understand the combustion process and soot formation of biofuels.In the simulation of combustion process of diesel engine,accurate and reliable chemical reaction mechanism can accurately describe fuel ignition and combustion process.Therefore,the development of chemical mechanism for multi-component diesel surrogate fuel,biofuel and soot formation is conducive to better study and understand the combustion and soot formation process of diesel and oxygenated biofuels.In this thesis,the reduced mechanisms for multi-component diesel surrogate fuel,acetone/n-butanol/ethanol(ABE),the C1-C5 alcohol and the formation of polycyclic aromatic hydrocarbons(PAHs)are proposed.The multi-dimensional numerical simulation method of CFD coupled chemical reaction kinetics is used to investigate the combustion and soot formation process of diesel and oxygenated fuels.The main work of this thesis is as follows:(1)Based on the open source programs KIVA-3V and CANTERA,the numerical simulation platform of CFD coupled chemical reaction kinetics was established and applied to the multi-dimensional simulation of the combustion process of diesel and oxygenated biofuels in this thesis.(2)A reduced chemical kinetics mechanism for toluene reference fuel(TRF),containing PAHs formation,was developed and coupled with the reduced mechanism of ABE to form a reduced mechanism of TRF-ABE-PAH,which consists of 98 species and 456 reactions.The experimental data including the ignition delay,premixed flame species profiles,jet-stirred reactor oxidation and direct injection combustion were taken to validate the reduced mechanism.The results show that the reduced mechanism can accurately predict igntion delays,species profiles and the combustion and emission on IC engine,indicating that it can be used to simulate the combustion process and emission formation of diesel and oxygenated biofuels.(3)The ignition characteristics of TRF-ABE fuels were simulated by using the proposed mechanism.The results show that the ignition delays of fuels show different variation patterns in different temperature ranges.In the high temperature zone,the ignition delays of fuels are slightly shortened with the increase of ABE blend ratio.In the middle temperature zone,the change of the ignition delays is relatively gentle.In the low temperature zone,there is a negative temperature coefficient zone,which is gradually weakened with the increase of ABE blend ratio.The pressure has a great influence on the ignition delays.As the pressure increases,the ignition delays are significantly shortened in a wide temperature range,especially in the mid-high temperature zone.(4)The formation process of PAHs in TRF-ABE fuel premixed laminar flame was simulated by using the proposed mechanism.With the increase of ABE blend ratio,the concentration of PAHs gradually decreases.A1 is the most abundant aromatic component of all fuels.The trends of A2,A3 and A4 are similar to that of A1,and the peak concentrations decrease in turn,and the positions are pushed backward.The concentrations of OH,O and H increase with the increase of ABE blend ratio,and the peak concentrations remain unchanged.The mole fraction of toluene decreases significantly due to the addition of ABE.In the main reaction zone,the concentration of toluene in the TRF laminar flame is still the highest,resulting in the highest amount of A1.(5)A reduced chemical kinetics mechanism for TRF-alcohols-PAH consisting of C1-C5 alcohols was developed,which contains 184 species and 784 reactions.Compared with the experimental data including ignition delay,premixed flame species profiles and direct injection combustion,the results show that the reduced mechanism can accurately describe the fuel ignition and combustion characteristics over a wide range of operating conditions.The predicted combustion and emission characteristics of diesel fuel and alcohol fuels in engines are in good agreement with the experimental results.(6)Based on the experiments of diesel and ABE in constant volume combustion bomb,a CFD model was established and coupled with the proposed mechanism to simulate the combustion and soot formation process of diesel-ABE fuel.The results show that,with the increase of ABE blend ratio,the combustion pressure and heat release rate decrease,the flame lift-off increases,the overall equivalence ratio decrease,PAHs and soot emissions reduce;as the initial ambient temperature decreases,the formation and oxidation of PAHs and soot are inhibited,the soot emission reduces;as the initial oxygen concentration decreases,the production rate of PAHs and soot increase,the OH mass decrease,so soot emission increases.(7)Based on the experiments of diesel and ABE fuel on diesel engine,the CFD model coupled with the proposed mechanism was applied to investigate the combustion and emission characteristics of diesel-ABE fuel.The results show that: with the increase of ABE blend ratio,the overall equivalence ratio decreases,the mixture becomes more uniform,the peak value of OH mass increases,soot and PAHs emissions decreases,NOX emission increases;with the advance of injection timing,the average peak temperature and the heat release rate increase,the production of OH mass increases,soot and PAHs emission decrease,NOX emission increases;with the increase of EGR rate,the oxygen concentration decreases,the chemical reaction rate decreases,the OH radical concentration decreases,so soot and PAHs emissions increase,NOX emissions decrease.